The present invention relates to a process for the synthesis of aryl alkyl ethers.
It relates more particularly to a process for the improved synthesis of aryl alkyl ethers by O-alkylation of the corresponding phenol compounds.
Aryl alkyl ethers are very useful intermediates, in particular for the preparation of dyes, plant-protection agents and fragrances. Their applications are reported in particular in Ullmann Enzyklopxc3xa4die der Technischen Chemie, Volume 13, pages 450-453 and Volume 14, pages 760-763. A great many preparation processes have consequently been provided.
Some consist in alkylating phenol derivatives with alkyl halides or alkyl sulphates, as indicated in Pure and Applied Chemistry, Volume 68, No. 2, pages 367-375 (1996). However, these processes exhibit numerous disadvantages. Some reactants, such as dimethyl sulphate, are highly toxic. Furthermore, the acid released during the reaction must be neutralized; in point of fact, some phenols are highly sensitive to the neutralizing agents.
Other processes use an alcohol, for example methanol, as alkylating agent. The reaction is carried out at very high temperatures of greater then 250xc2x0 C. In the majority of cases, this reaction is not selective and C-alkylation by-products are formed. When it is selective, the degree of conversion is low. Such a process is described in Volume 44 of Catalysis Today, pages 253-258 (1998).
Processes for alkylation by dialkyl carbonates were then envisaged, in particular processes for O-alkylation by dimethyl carbonate. The catalyst used is chosen from tertiary amine salts, diamines, quaternary ammonium salts or tertiary phosphines. Such processes are disclosed, for example, in U.S. Pat. No. 4,192,949. However, the reaction temperature, pressure and duration of these syntheses, carried out in a closed reactor, remain high, which is highly disadvantageous industrially.
In order to prepare aryl alkyl esters under milder conditions, the authors of the article which appeared in Synthesis, Volume 5, pages 382-383 (1986), recommend the use of potassium carbonate with a cocatalyst, crown ether 18-6. This is catalysis by solid/liquid phase transfer. The reaction is carried out at atmospheric pressure and at 100xc2x0 C. but the high toxicity and the high cost of the phase transfer cocatalyst, the crown ether, are major disadvantages. Furthermore, the mean rate of formation of ether per mole of catalyst is of the order of 0.03 mol per hour, which is low. Another catalytic system was then proposed, in order to operate with nontoxic reactants. This is still phase transfer catalysis. Such a process is described in Industrial and Engineering Chemistry Research, volume 27, pages 1565-1571 (1988). Tundo et al. use, as catalytic system, polyethylene glycol adsorbed on a solid stationary bed composed either of potassium carbonate or of xcex1-alumina beads. In the latter case, potassium carbonate is also adsorbed on the xcex1-alumina beads. This process exhibits the disadvantage of using a complex catalytic system with at least two constituents, polyethylene glycol and potassium carbonate. The simultaneous presence of these two constituents is essential in obtaining a good yield. This is because, by way of comparison, when the catalytic system is composed of xcex1-alumina beads covered solely with 5% by weight of potassium carbonate, the conversion of phenol to anisole, as indicated in FIG. 4, is 25%, which is very low. Furthermore, the heterogeneity of the reaction mixture reduces the efficiency of the reaction. The mean rate of formation of the ether per mole of catalyst is consequently only 0.13 mol per hour.
A similar continuous process was envisaged by Bomben et al. in an article which appeared in Industrial and Engineering Chemistry Research, Volume 38, pages 2075-2079 (1999). The authors of this article use, as catalytic system, a stirred catalytic bed composed of polyethylene glycol and of potassium carbonate. The disadvantages related to such a process remain still the complexity of the two-component catalytic system and the low value of the mean rate of formation of ether per mole of catalyst, which is only 0.7 mol/mol.h.
Another process, disclosed in Japanese Application JP 06145091, consists in reacting a phenol compound, such as phenol or hydroquinone, and an alkyl carbonate. The catalyst used is an alkali metal salt, in particular potassium carbonate. The reaction necessarily takes place in the presence of a nitrogenous organic solvent, such as pyridine, a formamide or an alkylacetamide. The authors have shown, in Comparative Example 1, that the yield was zero when the reaction was carried out without solvent. Furthermore, the reaction time remains long and the yields are low.
A person skilled in the art is therefore still looking for a process for the synthesis of aryl alkyl ethers which is inexpensive and selective, with a good yield, and for which the reaction conditions are mild, in particular as regards the pressure and the temperature.
Such a process is a subject-matter of the present invention.
The invention relates to a process for the synthesis of aryl alkyl monoethers by reaction of a phenol compound, comprising one or more hydroxyl groups attached to the aromatic cyclic system, and of a dialkyl carbonate, characterized in that the said process is carried out without solvent, at a pressure of between 0.93xc3x97105 Pa and 1.07xc3x97105 Pa, at a temperature of between 100xc2x0 C. and 200xc2x0 C., in the presence of a catalyst chosen from the group consisting of alkaline carbonates and alkaline hydroxides and in that the dialkyl carbonate is added gradually to the reaction mixture.
This process exhibits the advantage of being simple and inexpensive and makes it possible to obtain ethers with a very good yield.
This is because it is a solvent-free process using only a single catalyst. The use of a limited number of constituents thus reduces the cost. Furthermore, the operating conditions, in particular as regards the pressure and the temperature, are easy to implement industrially.
The reaction is selective. This is because, when the phenol compound comprises only one hydroxyl group on the aromatic cyclic system, only the corresponding aryl alkyl ether is obtained, without formation of by-products.
When the phenol compound comprises two or more hydroxyl groups attached to the aromatic cyclic system, an aryl alkyl monoether is predominantly obtained. Only small amounts of polyethers are obtained.
Another advantage of this process is the high value of the mean rate of formation of the ether with respect to the amount of catalyst used, which is of the order of 2 to 6 mol of ether formed per hour and per mole of catalyst used. With the already existing processes, the mean rate was less than or equal to 1 mol of ether formed per hour and per mole of catalyst.
This process makes it possible to selectively obtain a wide range of aryl alkyl monoethers starting from a dialkyl carbonate and a phenol compound.
The phenol compound is preferably chosen from the compounds of formula (I) 
in which R2, R3, R4, R5 and R6, which are identical or different, each represent
a hydrogen atom,
a substituted or unsubstituted, saturated or unsaturated, C1 to C20 alkyl radical,
a substituted or unsubstituted aryl or aralkyl group,
a halogen atom,
a nitrile or nitro group or a group of formula: 
xe2x80x83in which R7 is a C1 to C20 aliphatic radical, a C7 to C12 aralkyl radical or a C6 to C14 aromatic radical, R8 is a C1 to C20 aliphatic radical, a C7 to C12 aralkyl radical or a C6 to C14 aromatic radical, and R9 is a hydrogen atom,
it being possible for two adjacent radicals, for example R2R3 or R3R4 or R4R5 or R5R6, to be connected to one another to form a saturated or unsaturated aliphatic ring, an aromatic ring or a saturated or unsaturated heterocycle which are unsubstituted or substituted by the groups as described for R2 to R6. The substituents of the R2 to R6 radicals are chosen in particular from halogen atoms, nitrile or nitro groups, or groups of formula: 
xe2x80x83in which R7, R8 and R9 have the preceding meanings.
The compounds of formula I are known compounds which are commercially available or which are prepared according to known methods.
The preferred phenol compounds are phenol, p-cresol, 4-chlorophenol, 2-naphthol, 4-hydroxybenzophenone, 2,4-dihydroxybenzophenone and catechol.
The dialkyl carbonate is a carbonate for which the alkyl groups, which are preferably identical, are C1 to C4 groups. Preferably, the dialkyl carbonate is chosen from the group consisting of dimethyl carbonate and diethyl carbonate and better still the dialkyl carbonate used is dimethyl carbonate.
The amount of dialkyl carbonate used is generally between 0.9 and 5 mol and preferably from 1 to 2 mol with respect to the phenol compound.
The catalyst used is chosen from the group consisting of alkaline carbonates and alkaline hydroxides. The alkaline carbonates comprise both neutral alkaline carbonates and alkaline hydrogencarbonates.
Preferably, the alkaline ion is potassium and better still the catalyst used is neutral potassium carbonate.
The amount of catalyst used is between 0.01 and 0.1 mol per mole of phenol compound and preferably between 0.015 and 0.05 mol.
The reaction is carried out at a pressure of between 0.93xc3x97105 Pa and 1.07xc3x97105 Pa, i.e. at a pressure of between 700 mmHg and 800 mmHg. The local atmospheric pressure is generally within this range and the reaction is carried out at this pressure. The temperature is between 100xc2x0 C. and 200xc2x0 C. and preferably between 140xc2x0 C. and 180xc2x0 C.
It is a solvent-free process, that is to say a process without the introduction of a product other than the compounds of the reaction.
An implementation of the invention is now given.
The phenol compound, either completely or in part, the catalyst and optionally the final product, the already alkylated phenol compound, are placed initially in the reactor. The mixture is heterogeneous at ambient temperature. It is heated to a temperature of between 100xc2x0 C. and 200xc2x0 C., preferably of between 140xc2x0 C. and 180xc2x0 C. The mixture becomes homogeneous at the reaction temperature.
The dialkyl carbonate is then gradually introduced into the reaction mixture. Preferably, a small portion of the dialkyl carbonate is introduced into the mixture and then the dialkyl carbonate is introduced continuously with a flow rate of between 1 and 20 mol/h per mole of catalyst or by noncontinuous additions very close together in time.
When only a portion of the amount of phenol compound is present initially in the reactor, the other portion is added gradually to the reaction mixture, preferably simultaneous with the addition of dialkyl carbonate. Its flow rate is then preferably between 0.5 mol/h and 20 mol/h per mole of catalyst.
The alcohol produced during the reaction is distilled off as it is formed. The ether obtained can subsequently be purified by distillation.
This process is particularly well suited to the synthesis of aryl methyl monoethers, obtained by O-methylation of the corresponding phenol compound, by reaction of dimethyl carbonate with the phenol compound.
Very high yields, of greater than 95%, can be obtained by O-methylation, in particular starting from p-cresol, 4-chlorophenol and 2-naphthol.
Good yields are also obtained starting from phenol, 4-hydroxybenzophenone and 2,4-dihydroxybenzophenone.
This process is also particularly well suited to the synthesis of aryl ethyl monoethers, obtained by O-ethylation of the corresponding phenol compound, by reaction of diethyl carbonate with the phenol compound. Good yields are obtained, in particular starting from p-cresol.
The following examples illustrate, without implied limitation, alternative embodiments of the invention.